H-D exchange was observed in the methyl group during the formation of sulfones in the reaction of dimethyl sulfide-d(6) or thioanisole-alpha,alpha,alpha-d(3) with singlet oxygen in aprotic solvents. No exchange was observed in the sulfoxides obtained. The proton in the sulfones was shown to come from adventitious water, since the oxidation of C6H5SCH3 in the presence of D2O lead to the formation of sulfones with monodeuteriation. The O-16(2)-O-18(2) tracer study demonstrated no oxygen scrambling in the sulfones. All the results indicate that the sulfones are formed intramolecularly via an intermediate possessing one activated proton exchangeable with trace water, a suggested structure for which is S-hydroperoxysulfonium ylides (RS(+)(OOH)CH2-). Kinetic isotope effects (k(H)/k(D)=2-4) observed for methyl protons in the sulfone formation suggest that the rate-determining step is the intramolecular proton abstraction in the persulfoxides (RS(+)(OO-)CH3) generating S-hydroperoxysulfonium ylides. The conversion of the ylide intermediates to sulfones is shown to be facile on the basis of PM3 theoretical calculations.